The present invention relates to an adaptable pre-matched tuner system and method, and more particularly to such a system to be used in load-pull set-ups for the measurement, characterisation and testing of RF or microwave devices. It is particularly useful when devices presenting very high reflection factors have to be measured, such as high-power, low impedance transistors, diodes and MMICs, especially when operated in saturated mode.
Traditional large-band microwave tuners have been used for some time already to synthesize impedances within RF/microwave measurement set-ups. Their capability of synthesizing high reflection loads is however somewhat limited, which makes that in practice they cannot be used reliably when characterising the high-power, low-impedance devices that have appeared in the market during the recent years.
These limitations are mainly of two natures:
a) Power Limitations
In active tuners, the maximum handling capability is determined by the characteristics of the active circuitry inside, and is generally extremely low, usually below 1 Ampere.
In electromechanical tuners, the maximum handling capability is related to the connector current handling capability, as, at high reflection factors, very high currents are generated. Also, voltage limitations are also an issue as corona discharges can take place between the tuning slug and the central conductor at impedances at which the gap between the two becomes very small.
b) Accuracy Limitations
Even when power limitations are not a factor, traditional tuners, especially electromechanical ones, cannot generate, characterise and reproduce, accurately and consistently, reflection factors higher than approximately 0.90. Also, network analysers, which in some cases constitute an integral part of the calibration set-up, become less and less accurate when very high reflection factor loads (xcex93xe2x89xa70.95) are to be measured.
To overcome these inherent difficulties of traditional large-band tuners, a solution has already been proposed: an impedance transformer can be introduced between the DUT and the tuner (see FIG. 6), so as to reduce the reflection factor requirement at the tuner ports, by effectively shifting the impedance seen by the tuner into a Smith Chart area which it can cover adequately.
In fact, impedance transformers can be of different types, but up to now the ones that have been described in the literature are xcex/4 transmission lines, used when microstrip or stripline devices mounted on a test-jig have to be characterised, and pre-matching probes, when on-wafer measurements need to be performed.
These impedance transformers do sometimes work, but they are not always practical: they are inherently narrowband, they involve significant additional ohmic losses along the measurement set-up signal path and they cannot be adjusted. This means that a long and complicated trial-and-error process has to take place whenever a new device has to be characterised, or even when the measurement frequency is changed. Also, in practice, no phase control is possible. Finally, xcex/4 transmission lines might also prove cumbersome to implement because, for lower frequencies ( less than 500 MHz) and larger transformation ratios (more than 4:1), transmission lines become extremely long and wide.
It is an object of the present invention to provide an adaptable pre-matched tuner system and calibration method which resolves the above-noted deficiencies in the prior art. In accordance with the invention, this object is achieved with an adaptable pre-matched tuner system for measuring reflection factors above xcex93=0.85 for a DUT, comprising a first and second large-band microwave tuners connected in series, said first and second large-band tuners being mechanically and electronically integrated; and a controller for controlling the two large-band tuners. The first tuner is adapted to act as a pre-matching tuner and the second tuner is adapted to investigate an area of a Smith Chart that is difficult to characterise with a single tuner, so that the combination of the first and second large-band tuners permits the measurement of reflection factors above xcex93=0.85.
The pre-matched tuner system allows the generation of a very high reflection factor at any point of the reflection factor plane (Smith Chart). The pre-matched tuner must be properly calibrated, such as to be able to concentrate the search for optimum performance of the DUT in the exact location of the reflection factor plane where the DUT performs best, using a pre-search algorithm.